1. The detection and molecular cloning of yeast DNA sequences related to the v-sis oncogene were previously reported. A 1.5-kbp fragment (Xba I-Bam HI) encompassed the hybridizing region. Necleotide sequence analysis showed that the homology was localized in a stretch of 16 bp, with one mismatch. The cloned gene, therefore, does not belong to the sis family. However, it was found to be single copy gene in yeasts with coding capacity for a protein of about 20 kd. It was mapped to chromosome X of Saccharomyces cerevisiae, at about 7.5 cM from the centromere, linked to the ilv-3 gene. Gene disruption experiments proved that it is a nonessential gene, and may be a useful tool for the intergration of heterologous genes in the yeast genome. 2. By means of standard recombinant DNA techniques, a number of yeast expression vectors have been constructed. Yeast promoters such as ADC-1, PGK (constitutive), and GAL-1, GAL-10 and MF-Alpha (inducible) are currently used to express sis-related protein in yeasts.